This proposal focuses on the early development of attention- deficit/hyperactivity disorder (ADHD) and comorbid oppositional defiant disorder (ODD). Although these disorders are typically diagnosed in elementary school age children, associated symptoms are thought to appear frequently by the preschool years. By the time ADHD children are diagnosed and treated, the comorbid problems and interaction patterns that may have caused them are likely firmly entrenched and resistant to treatment. Early interventions that target children at risk for developing ADHD could have the potential to prevent the development of comorbid problems among ADHD children and may limit the symptom severity and degree of impairment among children with ADHD. Early identification of these children is critical to early intervention. Therefore, the proposed project seeks to identify factors that predict the development of pure ADHD and comorbid ADHD/ODD among preschool children with externalizing problems. This prospective study will follow 180 three-year-old children with externalizing problems and 70 non-problem comparison children. Diagnostic and family assessments will be conducted yearly from age three to age six. The following specific aims will be addressed: a) to determine the extent to which preschoolers with externalizing problem are later diagnosed with ADHD, b) to evaluate the utility of early hyperactivity/impulsivity in predicting later ADHD diagnoses, c) to examine whether the chronicity and biological markers of early hyperactive/impulsive symptoms and neuropsychological functioning predict later ADHD and whether they predict ADHD beyond early hyperactivity/impulsivity, d) to evaluate the utility of early hostility/defiance in predicting later ODD among ADHD children, e) to examine whether parental discipline, family stress (including parent psychopathology, marital discord, negative life events, SES, and single parenthood), and family history of antisocial behavior predict later ODD among ADHD children and whether they predict ODD beyond what would be predicted from early hostility/impulsivity, f) to examine whether parental discipline mediates the effects of family stress on the development of ODD among ADHD children, g) to evaluate whether family stress interacts with early hyperactivity/impulsivity and hostility/defiance in predicting parental discipline and later ODD, and h) To examine whether early hostility/defiance and hyperactivity/impulsivity interact with parenting in predicting later ODD.